1. Field of the Invention
The invention is directed to the provision of multiple voltages, and particularly to an efficient power management method and apparatus for providing multiple supply voltages from a single voltage source.
2. Description of the Related Art
In modern fourth generation wireless handset solutions, there is an expectation that a power management integrated circuit (PMIC) will generate an array of voltages of different values for powering various blocks, including for example digital cores, inputs/outputs, analogue circuits and power amplification stages. These blocks will have different voltage requirements. The voltages will be required to be generated from a single lithium ion cell having a terminal voltage with a typical value between 2.6V and 5.5V.
In order to provide this a so-called H-bridge buck-boost topology, as illustrated in FIG. 1, is typically provided.
With reference to FIG. 1, there is shown a voltage generation stage 100. A voltage source 110, typically a battery, provides an input voltage on line 112. The switching elements consist of a buck section 104 formed by switches 102 and 103, and a boost section 109 formed by switches 105 and 106. Capacitor 107 is a capacitive storage element and inductor 108 is an inductive storage element. The voltage source 110 has an exemplary voltage supply of 2.5V. Supply stage 100 has to switch between buck and boost modes if the output voltage is required to be greater than 2.5V.
In boost mode, the voltage source 110, typically a battery, has a value which is lower than a desired voltage at the output. In buck mode the voltage source has a value which is higher than a desired voltage at the output.
A problem with the topology such as illustrated in FIG. 1 is that a separate voltage generation stage 100 must be used for each voltage required. Thus the entire circuit of FIG. 1 must be replicated for each required voltage. This results in a number of buck-boost circuits, and an associated proliferation of inductors. This adds to cost, takes up space, and generates interference.
In order to overcome these problems, in the prior there has been proposed approaches to improve power management ICs. These approaches include: the provision of on-chip inductors; switched capacitor solutions; and multi-winding transformers.
By fabricating inductors on-chip it is possible to gain some miniaturisation. However on-chip inductors have a high series resistance that hampers their usage in high current supplies. Also, a number of supplies would demand considerable IC area.
Switched capacitors enable the creation of switched mode supplies without a single inductor. These generally work very well at low currents, but at high currents the value of capacitance required dominates the chip fabrication area. If off-chip capacitors are used, the track inductance between the on-chip switches and off-chip capacitors becomes a severe problem, and as most of the switched capacitor elements are floating then serious electromagnetic induction (EMI) is generated. Because of the number of switching elements required, efficiency is generally less than an inductor based buck-boost supply.
A prior art multi-winding transformer technique is a transformer-coupled forward or flyback converter, with multiple windings or taps that are rectified to give a set of DC voltages. However such an arrangement has a number of problems. It is difficult to independently regulate each supply. A specialised transformer needs to be wound, rather than being able to use off the shelf inductors. The arrangement is inflexible: if one of the supply voltages needs to be changed, this requires a transformer redesign.
It is an aim of the invention to provide an improved power management arrangement for the provision of multiple voltage levels.